In December 2004, Voyager 1 became the first spacecraft to pass the "termination shock" (the edge of the dark blue circle in the image at the link above). This is where the Sun's solar wind starts to dramatically slow down (to sub-sonic speeds), indicating a lessening of the Sun's influence. The termination shock is still well within the heliosphere, where the Sun's magnetic forces and solar wind outweigh the magnetic forces of the galaxy and the force of the galactic (i.e., interstellar) wind.

Ever since that time, Voyager 1 has been traveling through the the "heliosheath"--the part of the heliosphere between the termination shock and the "heliopause," which is the edge of the solar system. In recent months, Voyager researchers have noticed that the number of charged particles detected from outside the solar system is increasing dramatically and much more rapidly than before. In the most recent week of recorded data, these particles have increased 5%. In the most recent month, they have increased 9% total. In the 3 years before that, these interstellar particles have increase only a total of 25%. Something is changing now--and the change is getting faster and faster.

Researchers are estimating that Voyager 1 might be only weeks or months away from crossing the heliopause, where it will become the first human-made object to reach interstellar space. This should allow Voyager 1 to transmit back to us the first in situ measurements from there.

The timing of this passage could not be better. In 2015, the data recorder on board the spacecraft will stop being used because of power limitations. The nuclear fuel on board Voyager 1 will seriously taper off in 2020, severely limiting the power for the scientific instruments themselves. Around 2025, Voyager 1 should use up its nuclear fuel completely, and will lose contact with the Earth.

Voyager 1 has taken 34 years to go from the Earth to this point, traveling at approximately 38,000 miles per hour (61,155 kilometers per hour) after its encounter with Saturn in late 1980. It was going a bit slower than that before, but gravity assists from Jupiter and Saturn propelled it to that speed. The spacecraft is now 3.75 times the distance from the Sun that Pluto is.

The solar system might seem huge, taking 34 years or so to get to the edge of it--but that's just peanuts compared to the size of interstellar space. If Voyager 1 was pointed towards the nearest star, Proxima Centauri, (it's not, but for example's sake) it would take it approximately 76,443 additional years to reach it from its current position (assuming constant speed and assuming the stars stayed still). To get a sense of how long that is, modern humans were living alongside Neanderthals that long ago!

At any rate, probably at some point in the coming months we can claim to be a species with interstellar capabilities (super, super tenuous as it is). Still, it's pretty cool! Hopefully we'll learn some good stuff!

In about 40,000 years, Voyager 1 will drift within 1.6 light years (9.3 trillion miles) of AC+79 3888, a star in the constellation of Camelopardalis. In some 296,000 years, Voyager 2 will pass 4.3 light years (25 trillion miles) from Sirius, the brightest star in the sky . The Voyagers are destined—perhaps eternally—to wander the Milky Way.

My understanding is RTGs are not like batteries but more like candles that get lit when they get built, so there's no way to delay their startup and you're going to get the same amount of light regardless of if you have a use for it or not. But this is completely not my field.

In about 40,000 years, Voyager 1 will drift within 1.6 light years (9.3 trillion miles) of AC+79 3888, a star in the constellation of Camelopardalis. In some 296,000 years, Voyager 2 will pass 4.3 light years (25 trillion miles) from Sirius, the brightest star in the sky . The Voyagers are destined—perhaps eternally—to wander the Milky Way.

Yeah. According to wikipedia, that star (AC+79 3888) is Gliese 445, a red dwarf that is currently 17.6 light years away, but is one of the fastest stars approaching the Sun. In 40,000 years, Gliese 445 will only be 3.45 light years away from the Sun (closer than the Alpha Centauri system, which is about 4.36 light years away currently). Even then, Voyager 1 will be 1.6 light years away from the star, which is like, a huge amount.

Well, Voyager survived the radiation from our sun during its life. I have to imagine it can handle the interstellar medium. Although, Voyager would be more prone to getting hit with things such as gamma rays outside the heliosphere, correct?

If I had to bet money I would say it's going to be nothing but a slow cold death for voyager but that's boring and it's more fun to imagine since we really don't know.

What if once Voyager is truly out in the ISM its velocity begins to climb and climb giving us new insight into why the expansion of the universe is accelerating? Granted it doesn't have the same origin but this will be the first time we've had anything man made interact with space outside the solar system.

RTGs are not like batteries but more like candles that get lit when they get built, so there's no way to delay their startup

Correct.

Oversizing the RTGs is about the only thing to do, and even that only gets you so far (plus all the other associated drawbacks...).

Of course, the other thing to do would be to travel faster. That would get you farther while keeping the ~50 year limit on RTG power.

For example, back in the early 2000s, NASA had a proposed "Interstellar Probe" mission that would use a 300 meter (diameter? radius?) solar sail to accelerate to about 14 astronomical units (AU) per year. (An astronomical unit is the distance between the Earth and Sun, about 93 million miles/150 million kilometers.) It would reach 200 AU from the Sun in about 10 years, and continue functioning past 400 AU before the RTG that they were proposing for the craft stopped working. For comparison, Voyager 1 is currently at 120 AU (after 35 years), and is traveling at about 3.6 AU per year.

They also had a next-generation Interstellar Probe proposed that would be similar to the first, but would use a 1 km solar sail, and keep functioning out past 1,000 AU going about 20 AU per year.

The distance to the Alpha Centauri system is approximately 272,000 AU. This next-generation type of craft could reach the nearest star system in a brief 13,600 years or so.

If you could reach 10%+ the speed of light, you could potentially start to travel to external star systems and still have power using RTGs alone within a 50 year limit.

I always thought the concept of these was great, because within a 20-50 year timeframe we would gain a huge amount of detail about the interstellar space that surrounds our solar system, and further the technologies that might eventually lead to interstellar travel (like light sails and nuclear drives). From a scientific standpoint, you could justify the expenditures by evaluating the "galactic weather/environment" that our solar system is currently passing through, or will pass through over the next 50 to 100 years, and seeing how the changes in that environment affect the heliosphere, the Sun itself, and therefore, the Earth. Clever technologists would get us to go farther in less time, and we'd learn a ton about how the space surrounding our solar system alters our solar system.

Or you could "bet on tech". Put out a probe with those concentrated PV cells pulled by a solar sail. No other power source.

The probe should take 20-30 years to get to interstellar space. At that point, hope that we have built a laser that can target it from a distance and power the CPV on board. Then collect the data and send it back to Earth. In this case, the laser could be based on an orbiting platform -- perhaps around Jupiter, where it uses the huge magnetic field to supply power as it orbits.

For example, back in the early 2000s, NASA had a proposed "Interstellar Probe" mission that would use a 300 meter (diameter? radius?) solar sail to accelerate to about 14 astronomical units (AU) per year. (An astronomical unit is the distance between the Earth and Sun, about 93 million miles/150 million kilometers.) It would reach 200 AU from the Sun in about 10 years, and continue functioning past 400 AU before the RTG that they were proposing for the craft stopped working. For comparison, Voyager 1 is currently at 120 AU (after 35 years), and is traveling at about 3.6 AU per year.

They also had a next-generation Interstellar Probe proposed that would be similar to the first, but would use a 1 km solar sail, and keep functioning out past 1,000 AU going about 20 AU per year.

Voyager 1 is really slow then: the Earth is doing about 2π AU/yr

In 1987 NASA/JPL had proposed the TAU, fission/ion. Main mission would be to measure stellar parallaxes, after all with a baseline of 1000AU instead of 2AU you get Hipparcos on steroids and if you send it in the direction of galactic north or south you could then directly find the distance to any (sufficiently bright) star in the Galaxy to within 2% or better.

M31's about 800,000 parsecs away, so a 1000AU chord would subtend 1.25mas. Hipparcos managed about 0.6mas accuracy, so if you don't mind big error bars and can put a large enough scope on the TAU to cope with the low apparent magnitudes you could think about a direct measurement of the distance to M31 that would be distinguishable from infinity.

The area immediately around the solar system is actually kind of interesting. I think typically, people think it's completely empty space between here and nearby star systems, but there is actually a fair amount of differentiation in that space. For example, right now our solar system is traveling through what is known as the "Local Interstellar Cloud" and is on the edge a very, very empty region of space termed the "Local Bubble". Here's an image from a somewhat dated article on the local galactic environment (from 2000) that shows the 10 light years surrounding the Sun and the clouds and empty spaces in that area:

You have to imagine that within the Local Interstellar Cloud there are probably some pretty wide variations in density, energy, and activity, and sending out craft like Voyager 1 could give us some insights into what we are traveling through or will travel through in the near future. Well worth the investigation, I think.

RTGs are not like batteries but more like candles that get lit when they get built, so there's no way to delay their startup

Correct.

Oversizing the RTGs is about the only thing to do, and even that only gets you so far (plus all the other associated drawbacks...).

Or use a fission reactor. Those have the advantage of only being depleted (over relevant timescales) according to the power that's actually used, rather than depleting at a constant rate regardless of power used. They also have a much better energy density at higher power levels, which will be needed for stuff like high-powered radio transmitters that will be needed at extra-solar distances.

RTGs are not like batteries but more like candles that get lit when they get built, so there's no way to delay their startup

Correct.

Oversizing the RTGs is about the only thing to do, and even that only gets you so far (plus all the other associated drawbacks...).

Or use a fission reactor. Those have the advantage of only being depleted (over relevant timescales) according to the power that's actually used, rather than depleting at a constant rate regardless of power used. They also have a much better energy density at higher power levels, which will be needed for stuff like high-powered radio transmitters that will be needed at extra-solar distances.

Heavier than RTGs, yes, but not ridiculously so. They're easily within the limits of any current launch platform, not to say anything about proposed launchers like SLS or Falcon Heavy. Jupiter Icy Moons Orbiter was intended to carry one.

What about Stirling radioisotope generators? As far as I can tell they produce 4x the power for the same weight, and will last just as long if not longer because there aren't thermocouples that will decay. They are still undergoing testing.

What about Stirling radioisotope generators? As far as I can tell they produce 4x the power for the same weight, and will last just as long if not longer because there aren't thermocouples that will decay. They are still undergoing testing.

For a truly long-endurance interstellar mission, you'll still be limited by the half-life of your fuel, but in this case a stirling radioisotope generator using Americum-241 (half-life = 432 years) would probably work very well. That said, if you want to talk to your probe at that range, you'll probably still need a power source with a lot more output. I would guess that if we ever do launch a serious extra-solar probe, it will probably have both a fission reactor for high power operations and a radioisotope generator for idle cruise periods.

RTGs are not like batteries but more like candles that get lit when they get built, so there's no way to delay their startup

Correct.

Oversizing the RTGs is about the only thing to do, and even that only gets you so far (plus all the other associated drawbacks...).

Or use a fission reactor. Those have the advantage of only being depleted (over relevant timescales) according to the power that's actually used, rather than depleting at a constant rate regardless of power used. They also have a much better energy density at higher power levels, which will be needed for stuff like high-powered radio transmitters that will be needed at extra-solar distances.

Apparently americium-241 is an option being looked at for RTG's on account of plutonium-238 shortages. Has a quarter of the power density and needs thicker shielding (due to the decay of the decay products), but on the plus side has a 432 year half-life.

If you want to get really speculative then there's the possibility of using such metastable isomers as 166m1Ho (half-life 1200 years) which might perhaps be provoked into decay by lower-energy photons like 180m1Ta can be. Might have trouble getting a net energy output, but there is at least a bit of potential there for on-demand solid-state nuclear energy for a decades-long mission.

Apparently americium-241 is an option being looked at for RTG's on account of plutonium-238 shortages. Has a quarter of the power density and needs thicker shielding (due to the decay of the decay products), but on the plus side has a 432 year half-life.

I would guess that if we ever do launch a serious extra-solar probe, it will probably have both a fission reactor for high power operations and a radioisotope generator for idle cruise periods.

For a reactor that needs to last that long you probably don't want to be turning it on and off all the time.

They would probably include something like a nickel-hydrogen battery that could be recharged from the RTG to cover transients for the occasional telemetry dump, but I would imagine intense surveying/downlinking would either happen only under command or at predetermined intervals to manage the lifetime of the reactor. That said, both the Brayton cycle turbine and the Stirling linear alternator are supposed to be very robust.

Right, exactly. Or tune the reactor to be able to support continuous transmission and dump the excess as heat when not in use. It's a telescope though, they're always in use.

In the near term, I'm not sure what a telescope would be continuously surveying through the entire mission. I'd seriously doubt it would be something Hubble-class, because any telescope of that capability would be more cost efficient to keep in-system. If it's something to try and get a longer baseline for estimating distances to individual stars and local galaxies, then you don't need a very big scope for that, and you wouldn't actually want to use it until it's a good ways out, and that's when you'd want your reactor running.

Another instrument I thought would be a good candidate for such a mission would be a powerful radar. I dunno how powerful it would have to be, but it seems that if you could periodically turn it on and spot ~1km planetesimals out to about 2 or 3 AU or so, you could go a fair way to getting a handle on what the population of the Kuiper Belt and Oort clouds are, and if your software is smart enough it could use that data to use the telescope to make detailed optical observations (if possible, it'd be pretty dark and cold out there)

Another instrument I thought would be a good candidate for such a mission would be a powerful radar. I dunno how powerful it would have to be, but it seems that if you could periodically turn it on and spot ~1km planetesimals out to about 2 or 3 AU or so, you could go a fair way to getting a handle on what the population of the Kuiper Belt and Oort clouds are, and if your software is smart enough it could use that data to use the telescope to make detailed optical observations (if possible, it'd be pretty dark and cold out there)

Let's see... a 500m radius disk @2AU would subtend 7e-19 steradians and a 1m radius disk (for receiving) @2AU would subtend 2.8e-24 steradians. So you'd only get about 2e-42 of an omnidirectional signal back. So you'd get back 1 out of every 5e41 photons. With a wavelength of (say) 1cm, that's about 2e-23J per photon so you'd need about 1e19J of them to get one photon back. If your platform is floating out at 20AU/yr then you'd have (about) 2 months to generate that much energy in order to scan the 2AU radius bubble before you're in the next one, so you'd need a continuous power output of the order of a terawatt. I'm ignoring here that the reflected signal won't be omnidirectional and I'm also ignoring transmission/absorption losses and noise, but it just seems like too big a hill to realistically climb.

Let's see... a 500m radius disk @2AU would subtend 7e-19 steradians and a 1m radius disk (for receiving) @2AU would subtend 2.8e-24 steradians. So you'd only get about 2e-42 of an omnidirectional signal back. So you'd get back 1 out of every 5e41 photons. With a wavelength of (say) 1cm, that's about 2e-23J per photon so you'd need about 1e19J of them to get one photon back. If your platform is floating out at 20AU/yr then you'd have (about) 2 months to generate that much energy in order to scan the 2AU radius bubble before you're in the next one, so you'd need a continuous power output of the order of a terawatt.

That's an ouch. Though I did consider that such a probe would probably be assembled on orbit and could afford a quite large antenna array (if it's doubling as the high-gain antenna, it will likely need to be fairly large anyways, no?). Also, I don't think you need to take a continuous survey, rather, you'd be turning running the radar maybe 2 or 3 times a year until out past the Kuiper Belt, and then at a much lower rate past that.

Another instrument I thought would be a good candidate for such a mission would be a powerful radar. I dunno how powerful it would have to be, but it seems that if you could periodically turn it on and spot ~1km planetesimals out to about 2 or 3 AU or so, you could go a fair way to getting a handle on what the population of the Kuiper Belt and Oort clouds are, and if your software is smart enough it could use that data to use the telescope to make detailed optical observations (if possible, it'd be pretty dark and cold out there)

Let's see... a 500m radius disk @2AU would subtend 7e-19 steradians and a 1m radius disk (for receiving) @2AU would subtend 2.8e-24 steradians. So you'd only get about 2e-42 of an omnidirectional signal back. So you'd get back 1 out of every 5e41 photons. With a wavelength of (say) 1cm, that's about 2e-23J per photon so you'd need about 1e19J of them to get one photon back. If your platform is floating out at 20AU/yr then you'd have (about) 2 months to generate that much energy in order to scan the 2AU radius bubble before you're in the next one, so you'd need a continuous power output of the order of a terawatt. I'm ignoring here that the reflected signal won't be omnidirectional and I'm also ignoring transmission/absorption losses and noise, but it just seems like too big a hill to realistically climb.

You just broke my brain. Seriously...and I hate to ask...can someone else smarter than me check that math? Even if it's wrong, I'm impressed by the effort. If it's right, I bow to you. A Terawatt? Is that continuos (TWh) or pulsed...and if the latter could it be feasibly provided via stored energy (capacitance, for example) (oh, you said continuous...shit)

It seems to me like it makes a lot of sense to assemble deep space vehicles in space out of multiple components so that you aren't limited by what you can fit into a single payload. That could get you like a 10m receiving dish for that radar.

If it's something to try and get a longer baseline for estimating distances to individual stars and local galaxies, then you don't need a very big scope for that, and you wouldn't actually want to use it until it's a good ways out, and that's when you'd want your reactor running.

You might need a decent telescope to pick out individual stars in neighboring galaxies. How big is over my head.

Telescopes tend to be booked solid. Any decent telescope will find something to do to pass the time.

I'm sure there's also plenty of stuff that will become meaningfully paralaxable at different distances as the thing goes to increasing distances.

Let's see... a 500m radius disk @2AU would subtend 7e-19 steradians and a 1m radius disk (for receiving) @2AU would subtend 2.8e-24 steradians. So you'd only get about 2e-42 of an omnidirectional signal back. So you'd get back 1 out of every 5e41 photons. With a wavelength of (say) 1cm, that's about 2e-23J per photon so you'd need about 1e19J of them to get one photon back. If your platform is floating out at 20AU/yr then you'd have (about) 2 months to generate that much energy in order to scan the 2AU radius bubble before you're in the next one, so you'd need a continuous power output of the order of a terawatt.

That's an ouch. Though I did consider that such a probe would probably be assembled on orbit and could afford a quite large antenna array (if it's doubling as the high-gain antenna, it will likely need to be fairly large anyways, no?). Also, I don't think you need to take a continuous survey, rather, you'd be turning running the radar maybe 2 or 3 times a year until out past the Kuiper Belt, and then at a much lower rate past that.

The key is that if you want to use the radar for discovery rather than inspecting something you've already found, you have no idea which direction to point it in. You can beam your photons to reduce the power requirements, but only at the cost of restricting the volume you can search before your probe has left the vicinity. You can make the dish bigger to reduce the power requirements but even if it's as large as Arecibo you're still looking at several kilowatts to get just a handful of photons back.

Actually looking back at it I think I missed a factor of 4π there: the solid angle of the 1km target at 2AU is actually π*(500m)²/(2x1.49598e11m)² = 8.8e-18 steradians (and likewise for the return), but then the fraction of an omidirectional signal that is intercepted gets the 4π factor back so the 2e-42 part remains the right order of magnitude (with the caveats that the reflected signal won't be omnidirectional and ignoring transmission/absorption losses and noise).

As an alternative, consider Neptune, which has a diameter of about 50,000km and an apparent magnitude of about 8 where it is, about 30AU from the Sun. Scale that down to 1km (brightness factor 4e-10) and a up to 1000AU from the Sun (brightness factor 9e-4) and down to 2AU from the observer (brightness factor 2.25e2) so a 1km body with the same albedo as Neptune (and also face-on) 1000AU from the Sun would have an apparent magnitude at a distance of 2AU of about 8 + 2.5xlog10(4e-10 x 9e-4 x 2.25e2) ~ 33.2. The HST gives out at about 30, so that class of instrument could passively detect in visible light such a 1km object about 1AU away (make a little leap for the higher albedo you might expect of an Oort cloud object vs Neptune), waaaay out there. Of course you'd need to point it around sufficiently as well, but at least you wouldn't need to take implausible amounts of power with you.

That would however be complicated by the appreciable relative motion of the spacecraft (a need to track in order to integrate photons received over enough time to make a detection).

Megalodon wrote:

KD5MDK wrote:

That could get you like a 10m receiving dish for that radar.

His argument assumes 500m...

Actually that's for the radius of the target body; I took 1m as the radius of the receiver.

If it's something to try and get a longer baseline for estimating distances to individual stars and local galaxies, then you don't need a very big scope for that, and you wouldn't actually want to use it until it's a good ways out, and that's when you'd want your reactor running.

You might need a decent telescope to pick out individual stars in neighboring galaxies. How big is over my head.

Last week, NASA reported that its Voyager 1 spacecraft, the farthest spacecraft (operational or non-operational) from Earth, is getting very near to interstellar space.

I though the pioneer spacecraft were further away, but since they have been out of communication for at least a couple of decades now, they go in the non-operational category.

There is also this apocryphal story about a basketball that was launched up in an early rocket explosion. Supposedly the back of the napkin math said that it had enough velocity from the explosion that it could have escaped earth and was supposedly further out then the pioneer craft, not that anybody believes it actually made it through the atmosphere.

Last week, NASA reported that its Voyager 1 spacecraft, the farthest spacecraft (operational or non-operational) from Earth, is getting very near to interstellar space.

I though the pioneer spacecraft were further away, but since they have been out of communication for at least a couple of decades now, they go in the non-operational category.

While the Pioneers have been traveling for longer, the orbital mechanics of its interaction at Saturn left Voyager 1 with the highest specific energy and so it overtook Pioneer 10 in 1998.

Quote:

There is also this apocryphal story about a basketball that was launched up in an early rocket explosion. Supposedly the back of the napkin math said that it had enough velocity from the explosion that it could have escaped earth and was supposedly further out then the pioneer craft, not that anybody believes it actually made it through the atmosphere.

About a week ago, NASA released an update--it seems more rapid changes in the environment around Voyager 1. From the link below:

Quote:

Two of three key signs of changes expected to occur at the boundary of interstellar space have changed faster than at any other time in the last seven years, according to new data from NASA's Voyager 1 spacecraft.

For the last seven years, Voyager 1 has been exploring the outer layer of the bubble of charged particles the sun blows around itself. In one day, on July 28, data from Voyager 1's cosmic ray instrument showed the level of high-energy cosmic rays originating from outside our solar system jumped by five percent. During the last half of that same day, the level of lower-energy particles originating from inside our solar system dropped by half. However, in three days, the levels had recovered to near their previous levels.

A third key sign is the direction of the magnetic field, and scientists are eagerly analyzing the data to see whether that has, indeed, changed direction. Scientists expect that all three of these signs will have changed when Voyager 1 has crossed into interstellar space. A preliminary analysis of the latest magnetic field data is expected to be available in the next month.